Imported Dengue --- United States, 1999 and 2000

Dengue is a mosquito-transmitted acute viral illness caused by any of the four dengue virus serotypes (DEN-1, DEN-2,
DEN-3, and DEN-4). Dengue is endemic in most tropical and subtropical areas of the world and has occurred among U.S.
residents returning from travel to such areas. CDC maintains a laboratory-based passive surveillance system for imported dengue
among U.S. residents (laboratory-diagnosed dengue in a U.S. resident living in an area without known authochthonous
dengue transmission, with travel history outside the United States in the 14 days before symptom onset). The system relies on reports
by clinicians to state health departments, which forward patient specimens to CDC for diagnostic testing. This report
summarizes information about imported dengue cases among U.S. residents during 1999--2000. The findings indicate that dengue
continues to cause disease in U.S. travelers abroad. Travelers to tropical areas should protect themselves from mosquito bites, and
health-care providers should consider dengue in the differential diagnosis of illness for patients who have returned recently from
such areas.

Serum samples from 216 persons who had suspected dengue
on the basis of clinical presentation and onset of symptoms
in 1999 and 2000 were submitted to CDC from 34 states and the District of Columbia
(1). From these samples, 41 (19%) cases were laboratory-diagnosed as dengue, of which
38 (93%) had IgM antibody or single high titers of IgG antibody in serum
samples, and three (7%) patients had isolation of dengue virus (DEN-2, DEN-3, and DEN-4; one case
each) (Table 1). Dengue diagnosis was negative in 112 (52%) patients, and indeterminate among 63 (29%) patients because convalescent samples for serologic
testing were unavailable.

Of the 40 laboratory-diagnosed dengue cases with available data, 22 (55%) were males. Age was reported for 35
persons (median: 37 years, range: 5--72 years). Clinical information was available for 28 patients with laboratory-diagnosed
dengue. The most commonly reported symptoms were fever (100%), headache (64%), rash (54%), and myalgia (39%). At least
three patients were identified as having been hospitalized, and one of these died (a male aged 41 years who had recently
returned from Bangladesh).

States reporting the highest number of cases were Massachusetts (four) in 1999 and New York (five) in 2000.
Travel histories within the 2 weeks before illness, available for 33 persons, indicated that infections probably were acquired in
Asia (13 cases), the Caribbean islands (12), Central America (seven), South America (one), and Africa (one). One patient
reported traveling both in the Caribbean islands and South America.

Data for both 1999 and 2000 indicated a marked decline in persons tested and in the percentage of persons
laboratory-diagnosed with dengue, compared with 1997 and 1998, when 349 persons were tested and 143 (41%) were
laboratory-diagnosed with dengue (2).

Editorial Note:

Dengue is transmitted to humans by
Aedes mosquitoes. The majority of U.S. residents with dengue
become infected during travel to tropical areas, although autochthonous transmission of dengue was documented in Texas in
1999 (3,4), and Hawaii in 2001
(5). The 1999 Texas outbreak was limited in scope because environmental factors (e.g.,
air conditioning) limited contact with Ae.
aegypti mosquitoes. In Hawaii, the outbreak might have been limited because
the vector was Ae. albopictus, a less efficient vector for dengue than
Ae. aegypti, which was not detected in localities where
dengue transmission occurred.

The incubation period of dengue has a range of 3--14 days (in the majority of cases, 4--7 days). Dengue virus infection
can be asymptomatic or cause illnesses ranging from mild undifferentiated fever to severe disease, including
hemorrhagic manifestations and shock (6). Dengue hemorrhagic fever (DHF) is characterized by fever, minor or major
bleeding phenomena, thrombocytopenia (<100,000
platelets/mm3), and evidence of increased vascular permeability
(e.g., hemoconcentration [hematocrit increased by
>20% from baseline], pleural or abdominal effusions, or hypoproteinemia)
(6). Dengue shock syndrome (DSS) is DHF with signs of circulatory failure, including narrow pulse pressure
(<20 mmHg), hypotension, or shock, and might result in a case fatality rate of approximately 10%
(7).

During 1993--1998, the number of imported laboratory-diagnosed U.S. cases increased, reflecting the impact of travel
and the occurrence of epidemic dengue in 1995 and 1998, especially in the Caribbean and Central America. The smaller
number of laboratory-diagnosed cases of dengue in 1999 and 2000 is temporally associated with a decreased number of cases
of dengue/DHF in the Americas (8).

The findings in this report are subject to at least two limitations. First, travel histories and clinical information
were available only for certain persons with dengue, and they might not be representative of all persons with imported
dengue. Second, the number of dengue cases referred to CDC for diagnosis represents a minimum estimate of the actual number
of U.S. travelers with dengue fever or DHF/DSS. Diagnostic samples might not have been sent for testing or might have
been sent to other laboratories. Because dengue is not nationally reportable, persons with suspected dengue might not be
reported. For example, in response to the 1999 documented authochthonous transmission of dengue, Texas implemented a
state laboratory-based active surveillance system. Of the 462 patients whose specimens were submitted for testing at the
Texas Department of Health (TDH) or other reference laboratories that year, 66 (14.3%) were laboratory-diagnosed as
dengue (TDH, surveillance data, cases not included in this report).

Persons traveling to areas where dengue is endemic should avoid exposure to mosquitoes by using repellents,
wearing protective clothing, and remaining in well-screened or air-conditioned areas. No vaccine is available for preventing
dengue infection. Health-care providers should consider dengue in the differential diagnosis of illness for all patients who have
fever and a history of travel to tropical areas within 2 weeks before the onset of symptoms. Supportive measures should
be administered, and only acetaminophen is recommended for management of pain and fever. Acetylsalicylic acid (i.e.,
aspirin) and other nonsteroidal anti-inflammatory agents are contraindicated because of their anticoagulant properties.
Dengue patients should be monitored for signs of DHF, especially hypotension, because prompt fluid therapy reduces morbidity
and mortality. Acute-phase (0--5 days after onset of symptoms) and convalescent-phase (6--30 days after onset of
symptoms) serum samples obtained for viral isolation and diagnosis should be sent through state or territorial health departments
to CDC's Dengue Branch, Division of Vector-Borne Infectious Diseases, National Center for Infectious Diseases, 1324
Calle Cañada, San Juan, PR 00920-3860, telephone 787-706-2399, fax 787-706-2496. Serum samples should be accompanied by
a summary of clinical and epidemiologic information, including date of onset of disease, date of collection of sample, and
a detailed recent travel history.

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